Chapter 11 The Autonomic Nervous System and Its Central Control
Upon completion of this chapter the student should be able to answer the following questions: 1.
What are the major cell types of the central and peripheral nervous systems?
2.
What are the major components of a neuron, and what are their functional roles?
3.
What are the functional roles of the major glial cell types?
4.
What are the main divisions of the central nervous system?
5.
How and where is the cerebrospinal fluid formed, and how does it circulate and exit the ventricular system?
6.
How is axon transport related to the response of the axon to transection?
he nervous system is a communications and control network that allows an organism to interact rapidly and adaptively with its environment, where environment includes both the external environment (the world outside the body) and the internal environment (the components and cavities of the body). To carry out its function the nervous system takes in sensory information from a variety of sources using specialized sensors (receptors), integrates this information with previously obtained information stored as memories and with the intrinsic goals and drives of the organism that have been embedded in its nervous system through evolution, decides on a course of action, and then issues commands to the effector organs (muscles and glands) to execute the chosen behavioral response.
Moreover, almost all behavioral responses require the coordination of many body parts. For example, even a simple reaching movement of the arm may require coactivation of axial muscles and possibly muscles in the lower extremity to maintain posture and balance, which themselves may be monitored by up to three different sensory systems (vision, vestibular, and proprioceptive) whose information has to be integrated. Furthermore, movements can alter the internal environment and thus can require compensatory changes in heart and breathing rates, blood vessel diameters, and other internal processes. All these variables are monitored and controlled by various specialized subsystems of the nervous system, all of which must work together for the organism to perform movements and more generally to survive. The succeeding chapters will describe these major subsystems individually; however, it should be remembered that in reality their activity is integrated to generate normal behavior.
To begin, it is useful to divide the nervous system into central and peripheral parts. The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of nerves and ganglia (small groups of neurons) that innervate all parts of the body and provide an interface between the environment and the CNS. The transition between the CNS and PNS occurs on the dorsal and ventral rootlets near to where they emerge from the spinal cord and on the cranial nerve fibers near to where they arise from the brain.
Cellular Components of the Nervous System
The nervous system is made up of cells, connective tissue, and blood vessels. The major cell types are neurons (nerve cells) and glia (neuroglia = “nerve glue”). In its most general form a neuron’s function can be defined as generation of signals (to be sent to other neurons or effector cells [e.g., muscle cells]) based on an integration of its own electrical properties with electrochemical signals from other neurons. The points where specific neuron-to-neuron communication occurs are known as synapses, and the process of synaptic transmission is critical to neuronal function (see ). Neuroglia, or just glia, traditionally have been characterized as supportive cells that sustain neurons both metabolically and physically, isolate individual neurons from each other, and help maintain the internal milieu of the nervous system; however, it is now known that they also have important roles in shaping the flow of activity through the nervous system.
The typical neuron consists of three main cellular compartments: a cell body (also referred to as a perikaryon or